The use of an excimer laser to reshape the cornea is now a widely practiced procedure for correcting myopia, astigmatism and hyperopia. As is known, the laser beam is shaped at a point in its pathway or scanned over the cornea to remove tissue by ablation. One of the most serious problems that can occur in any keratorefractive procedure that involves the reshaping of the central cornea is the decentration of the central region of the reshaped cornea from the point where the visual axis intersects the cornea. Such decentration is a known problem with excimer laser keratectomy including but not limited to PRK and LASIK surgery. To overcome the problem, excimer lasers conventionally provide a fixation light. The fixation light is positioned to be centered in the excimer laser beam above its last turning mirror. The optical axis of the fixation light is presumably aligned with the optical axis of the excimer laser allowing the surgeon to align the visual axis with the optical axis of the fixation light and thus presumably the optical axis of the excimer laser. However, since the fixation light can be viewed by the patient's eye when it is in a position other than directly aligned with the excimer laser beam, the ablated area may be decentered with the visual axis intersecting an area of the ablation other than at its center. This introduces astigmatism, irregular astigmatism, hyperopia, decreased visual acuity, "ghost" images and slows the postoperative recovery of vision.
Experience with excimer lasers has shown that a patient can observe the fixation light by rotation of his or her eye so that the eye's visual axis is not parallel to the laser beam. See FIG. 2. Such rotation will result in decentration of the ablation of the cornea. Accordingly, there is a need for a way to enhance centration of the cornea ablation laser with the visual axis of the eye.
Customarily, the centration of the excimer beam is accomplished by centering the pupil in the path of the excimer ablation either by marking the center of the pupil with two coincident visible laser beams which intersect at the center of the laser ablation or by positioning the pupil so that it falls inside the circle of a reticle in the eye piece of the surgeon's microscope, that reticle having previously been aligned with the excimer laser beam. The patient is then asked to look at a fixation light which has been previously calibrated to set it in the center of the excimer laser beam when the patient's eye is centered under the operating microscope. The problem with this fixation technique is that the patient's visual axis may be directed at the fixation light even when the pupil and eye are not centered in the excimer ablation beam because the eye is tilted, since the patient can still see the fixation light even when the eye is not properly centered.
Put differently, positioning the patient's eye under the laser so that the laser beam for ablating the cornea is centered symmetrically around the visual axis is essential for good optical quality of the corneal ablation. However, no matter how precisely the visual axis is lined up with a conventional fixation light, the laser's optical centration cannot be accurate unless the fixation light itself is located so that it is viewed by the patient in the center of the laser beam. Since the patient's fixation is subjective and the laser beam is invisible, assurance that the instrument has been calibrated so that the fixation light is perfectly centered with an excimer light cannot be obtained with sufficient certainty. The present invention provides assurance of proper alignment and centration between the laser beam and the patient's visual axis and allows the surgeon to align the fixation beam if necessary.
Accordingly, the present invention is directed to a method of assuring centration of the laser beam along the patient's visual axis.